In a display system, one or more light sources are driven by a driving circuit to illuminate a display panel. For example, in a liquid crystal display (LCD) system with light-emitting diode (LED) backlight, an LED array is used to illuminate an LCD panel. An LED array usually includes one or more LED strings, and each LED string includes a group of LEDs coupled in series.
FIG. 1 illustrates a block diagram of a conventional driving circuit 100. The driving circuit 100 is used to drive an LED string 106 and includes a converter circuit 102, a switch controller 104, and a switching regulator 108. The converter circuit 102 receives an input voltage VIN and provides an output voltage VOUT on a power line 141 to the LED string 106. The switching regulator 108 includes an inductor L1 coupled to the LED string 106 in series. The switching regulator 108 further includes a switch S1 and a diode D1 for controlling an inductor current flowing through the inductor L1. More specifically, the switch controller 104 provides a pulse-width modulation (PWM) signal 130 to turn the switch S1 on and off. When the switch S1 is turned on, the diode D1 is reverse-biased and the inductor current sequentially flows through the power line 141, the LED string 106, the inductor L1, the switch S1, and the resistor RSEN. The output voltage VOUT powers the LED string 106 and charges the inductor L1. When the switch S1 is turned off, the diode D1 is forward-biased and the inductor current sequentially flows through the inductor L1, the diode D1, the power line 141, and the LED string 106. The inductor L1 is discharged to provide power to the LED string 106. As such, by adjusting a duty cycle of the PWM signal 130, an average level of the inductor current is regulated and thus the current through the LED string 106 is regulated.
However, when the switch S1 is off, the voltage at the anode of the diode D1, e.g., VANODE, is increased to be greater than VOUT to forward bias the diode D1. Then, the voltage across the switch S1, e.g., VANODE−VR, is approximately equal to VOUT. When the switch S1 is on, the voltage across the diode D1 is approximately equal to VOUT. Therefore, the voltage ratings of switching elements such as the switch S1 and the diode D1 have to be greater than VOUT. Otherwise, the switching elements can be damaged when the operating voltages are approximately equal to VOUT. When the number of LEDs in the LED string 106 is increased to achieve a higher brightness, the output voltage VOUT is increased. As such, the switching elements with relatively high voltage ratings increase the power consumption and the cost of the driving circuit 100.